Question: I'd like to build a small greenhouse to extend my growing season. Can I also use it to start cool season crops in late summer for a fall harvest?

Answer: Greenhouses are great for extending the gardening season. You'll be able to harden things off earlier in the spring, and will be able to grow lots of seedlings outdoors, instead of crowded under fluorescent lights indoors. I would hesitate to start much in the greenhouse in the summer, because temperatures in there can get so hot. Most seedlings grow best at temps of 65-70 or so--hot temperatures can cause leggy growth and stunt the plants. One important thing for you to consider is how you will ventilate your greenhouse. It can get very hot, very quickly, even on a sunny spring day. If you're around during the day to monitor the temperature, you can do open the vents manually--just set a thermometer inside and when it gets above about 70F, crack the vents. If you aren't available to do this daily, then you might want to invest in a solar-powered vent opener, which will automatically open and close a vent at whatever temperature you set it at.